The spiral appears to have arms of green, blue, and pink/magenta - but what appears to be blue and what appears to be green are actually identical colors, confirmed by spectrometry.

The reason they look [like] different colors is because our brain judges the color of an object by comparing it to surrounding colors. In this case, the stripes are not continuous as they appear at first glance. The orange stripes don’t go through the "blue" spiral, and the magenta ones don’t go through the "green" one.

If you don't believe it, see the closeup view at Bad Astronomy. Link - via j-walkblog